Numerous processes are known for the separation of air by cryogenic distillation into its constituent components. Typically, the air separation process involves removal of contaminant materials such as carbon dioxide and water from a compressed air stream prior to cooling to near its dew point. The cooled air then is cryogenically distilled in an integrated multi-column distillation system having a high pressure column, a low pressure column and a side arm column for the separation of argon. The side arm column for the separation of argon typically communicates with the low pressure column in that an argon/oxygen stream containing about 8-12% argon is removed and cryogenically distilled in the side arm column. A waste nitrogen stream is generated to control nitrogen purity, U.S. Pat. Nos. 4,871,382; 4,836,836 and 4,838,913 are representative.
Recent attempts to improve the argon recovery at reduced power costs involved the use of structured and other forms of packing in the lower section of the low pressure column. The packings minimize pressure drop in the low pressure column and thereby take advantage of the increased relative volatility between nitrogen and argon at low pressure, thereby minimizing power consumption, as compared to column performance where trays are used as the vapor-liquid contact medium. U.S. Pat. No. 4,836,836 is representative.
One type of the more conventional cryogenic air separation processes calls for the operation of the low pressure column at a pressure ranging from about 14-20 psia, with the side arm column for argon separation operating at slightly lower pressure. The pressure utilized in the lower pressure column is such that nitrogen and argon product specifications can be met with maximum recovery of the components. Operating pressure is also indicative of power consumption in the cryogenic distillation process and is a major concern; operating pressures are selected to minimize power consumption. Therefore, the overall process design focuses on product specification, product recovery and power consumption.
Conventional multi-column system processes generate low pressure (15-20 psia) nitrogen product streams at high recovery while permitting efficient separation of argon. Recently there has been increased interest in generating moderate pressure nitrogen from a cryogenic distillation process, because of increased demand for inert atmospheres and enhanced oil recovery. Moderate pressure, e.g., pressures ranging from about 25-80 psia nitrogen, are generated by operating the low pressure nitrogen column at higher pressures than are utilized in conventional cryogenic air separation. The increased pressure in the low pressure column creates a problem with respect to the separation of argon from oxygen and nitrogen, because the relative volatility between argon and oxygen and between nitrogen and argon is reduced, thus making recovery of argon more difficult. The advantage achieved by low pressure column operation where the relative volatilities between argon and oxygen, and nitrogen and argon are large are reduced when this system is adapted by increasing the pressure of the low pressure column to moderate pressure inhibiting separation of the oxygen and nitrogen from the argon, and therefore recovery of argon, is lost.
One approach for producing moderate pressure nitrogen with high argon recovery is set forth in U.S. Pat. No. 4,822,395. That approach involves, inter alia, driving the argon column top condenser with the low pressure column bottoms as opposed to conventional processes wherein the argon column condenser is driven with the bottoms from the high pressure column. By utilizing the low pressure column bottoms to drive the argon column top condenser, a greater amount of high pressure bottoms may be used to provide reflux to the low pressure column. The introduction of the high pressure bottoms as reflux to the low pressure column at a point above the argon withdrawal point to the side arm column forces the argon downward toward the withdrawal point thereby enhancing recovery of argon from the system.